1. Field of the Invention
The present invention relates generally to the generation of hydrogen gas, such as for use in a fuel cell.
2. Background of the Related Art
A fuel cell is an energy conversion device that efficiently converts the stored chemical energy of a fuel into electrical energy. A proton exchange membrane (PEM) fuel cell is a particular type of fuel cell that generates electricity through two electrochemical reactions that occur at the proton exchange membrane/catalyst interfaces at relatively low temperatures (typically <80° C.). A necessary step in the operation of such fuel cells is the electrochemical oxidation of a fuel, typically hydrogen gas, to produce water. Therefore, finding a convenient source of hydrogen is necessary for the operation of a fuel cell.
The hydrides of some of the lighter metallic elements have been considered as a source of hydrogen for a fuel cell because they possess high concentrations of hydrogen that can be released by hydrolysis. Table 1 lists a number of hydrides of elements from the first and second groups of the periodic table that are useful for hydrogen generation, although the list is not meant to be exhaustive of all hydrides suitable for use in a hydrogen generator. The hydrides in Table 1 are divided into groups of salt-like hydrides and covalent hydrides. Table 1 provides the hydrogen content of each of the neat compounds as well as the hydrogen content of each of the compounds with sufficient water to hydrolyze the neat compound to hydrogen and oxide products, and with sufficient water to hydrolyze the neat compound to hydrogen and hydroxide products.
TABLE 1Hydrogen Content of Metal HydridesWt % H2With StoichiometricDouble StoichiometricCompoundNeatH2OH2OSalt-like HydridesLiH12.6811.897.76NaH4.206.114.80KH2.514.103.47RbH1.172.111.93CsH0.751.411.33MgH27.669.096.47CaH24.796.715.16Covalent HydridesLiBH418.5113.958.59Na BH410.6610.927.34K BH47.478.966.40Mg (BH4)211.9412.798.14Ca (BH4)211.5611.377.54LiAlH410.6210.907.33NaAlH47.478.966.40KAlH45.757.605.67Li3AlH611.2311.217.47Na3AlH65.937.755.76
The hydrides of the salt-like group continue to react and generate water as long as water is present. In some cases, the reaction products may form a “blocking layer” that slows or stops the reaction by blocking access of the water to the hydride. However, by breaking up or dispersing the blocking layer, the water can again contact the hydride and the reaction immediately returns to its initial rate. By contrast, some of the covalent hydrides react with water only to a limited extent, forming metastable solutions. Fortunately, the decomposition of these hydrides can be accelerated with catalysts so that, in the presence of catalysts, these covalent hydrides react similarly to the salt-like hydrides.
Some examples of hydrolysis reactions of light metal hydrides are shown in Table 2. The hydrogen yields shown in Table 2 are based upon the total mass of the hydrides and the water required for hydrolysis but do not take into account the mass of the hydrogen generator container. When considering the hydrogen yield from a complete hydrogen generator system, the mass of the container must also be taken into account. However, the container for a hydrogen generator that operates at low pressure can be quite light and therefore, the yields from a light weight hydrogen generator may approach the yields shown in Table 2. Table 2 provides the hydrogen yield for the stoichiometric amounts of reactants and the hydrogen yield from the reaction with twice the stoichiometric amount of water supplied.
The reactions shown in Table 2 include two or three hydrolysis possibilities for each of four metal hydrides. The first set of reactions show the ideal case, where the product is hydrogen and a metal oxide (e.g., MBO2). These reactions generally occur only at elevated temperatures. The second set of reactions show the reaction producing a metal hydroxide (e.g., MB(OH)4) although extra water beyond the amount listed in the first column is generally required to achieve complete hydrolysis, even to the hydroxide. The third set of reactions show the expected result from the hydrolysis of these compounds to the stable hydroxide hydrates as the products. The hydroxide hydrate is often the thermodynamically favored product. The effect of this thermodynamics is readily apparent from the comparison, for example, of Equation 10 with Equation 4. (See Table 2).
TABLE 2Hydrogen Yield from the Hydrolysis of Metal HydridesReactionHydrogen Yield (wt %)EquationStoichiometricDoubleNo.WaterWaterReaction to OxideLiBH4 + 2 H2O → LiBO2 + 4 H2113.958.592 LiH + H2O → Li2O + 2 H2211.897.76NaBH4 + 2 H2O → NaBO2 + 4 H2310.927.34LiAlH4 + 2 H2O → LiAlO2 + 4 H2410.907.33Reaction to HydroxideLiBH4 + 4 H2O → LiB(OH)4 +58.594.864 H2LiH + H2O → LiOH + H267.764.58NaBH4 + 4 H2O → NaB(OH)4 +77.344.434 H2LiAlH4 + 4 H2O → LiAl(OH)4 +87.334.434 H2Reaction to Hydrate ComplexLiH + 2 H2O → LiOH•H2O + H294.582.522 LiAlH4 + 10 H2O →106.303.70LiAl2(OH)7•H2O + LiOH•H2O +8 H2NaBH4 + 6 H2O →115.493.15NaBO2•4 H2O + 4 H2
Each of the reactions shown in Table 2 has both advantages and disadvantages as a source of hydrogen. The hydrolysis of lithium borohydride (LiBH4) to an oxide, as shown in Equation 1, produces the highest yield of hydrogen of any of the reactions shown, but only proceeds at high temperature. The hydrolysis of NaBH4 produces nearly as much hydrogen (Equation 3), but uses a less costly starting material. At lower temperature, the hydrolysis reaction of NaBH4 as shown in Equation 7 dominates, but one of the reaction products, NaB(OH)4, is very basic. Since the BH4− ion is normally stable towards hydrolysis at high pH, the rate of hydrolysis and the resultant hydrogen generation is reduced by several orders of magnitude in a high pH system.
However, in U.S. Pat. No. 6,534,033 and U.S. Patent Application Pub. No. US 2003/0009942, Amendola, et al. disclosed that a ruthenium catalyst catalyzes the decomposition of BH4− to hydrogen and borate even in a high pH system having added NaOH. Amendola disclosed that an aqueous solution of NaBH4 pumped over a catalyst bed produced a controlled hydrogen gas flow. The disclosed catalyst was 5% Ru on an unspecified ion exchange resin. The generation of gas was stopped by stopping the flow of the aqueous solution and restarted by restoring the flow.
In U.S. Patent Application Publication No. 2003/0014917, Rusta-Sallehy, et al. disclosed a system to generate hydrogen by using a chemical hydride in solution and contacting the solution with a catalyst to generate hydrogen. The disclosed process required that the borohydride be present as a solution and also required a pump. Both Rusta-Sallehy and Amendola disclosed systems that used sodium borohydride solutions to generate hydrogen but both have several significant limitations. The solutions required a substantial excess of water that decreased the mass yield of hydrogen. The processes also required pumps, which add to the weight and complexity of the systems. In addition, the aqueous solution is not completely stable. Even under basic conditions, the borohydride gradually hydrolyzes, thereby limiting the shelf-life of the chemical hydride solution.
The hydrolysis of lithium hydride (LiH) also has a high yield if it proceeds to completion as shown in Table 2, but the stability of lithium hydroxide hydrate makes it the stable end product, with a lower hydrogen yield, as shown in Equation 9. As reported in Proc. 39th Power Sources Conf., 184-187 (2000), Breault and Rolfe have shown that when this reaction is carried out in a water starved mode, the reaction proceeds to a mixture of Li2O and LiOH, with a hydrogen yield of over 8 wt %. However, this water-starved condition was achieved by injecting water throughout the mass of hydride in a slow, controlled manner using a complex mechanical control system, thereby substantially reducing the wt % yield of hydrogen from the generator system.
Storing sodium borohydride as a solution for use as a hydrogen source has been disclosed by Tsang in U.S. Patent Application Pub. 2003/0228505. Tsang disclosed metering an aqueous sodium borohydride solution over a ruthenium supported catalyst to generate hydrogen. To overcome the limitations of both reactivity and stability, Tsang disclosed storing the sodium borohydride prior to use in a solution having 5-40 wt % alkali hydroxide or alkaline metal hydroxide. At these very high pH levels, Tsang disclosed that sodium borohydride may be stored in solution for at least 6 to 12 months since the high pH renders the borohydride essentially non-reactive even in the presence of catalyst.
Tsang further disclosed mixing the high pH solution with water just before passing the solution over the supported catalyst in the hydrogen generator. Mixing with water brought the concentration of the high pH borohydride solution into the “reactive” range, which Tsang disclosed is less than about 10 wt % strong base. While Tsang disclosed the desirability of having high concentrations of borohydride in the solution passing over the supported catalyst, the final mixed solution was disclosed as being between 5 and 15 wt %. Tsang noted that the maximum solubility of sodium borohydride in water at room temperature is about 55 wt %. Tsang further disclosed that the best mode practice was to meter the two solutions with two different pumps and mix the solutions just upstream of the supported catalyst. The system and methods disclosed by Tsang do not address or solve the problems of making a light weight hydrogen generator because the two required pumps and the hydroxide necessary for storing the borohydride solution add significant weight to the disclosed hydrogen generator.
Weight is a characteristic of electrochemical cells generally, and fuel cells in particular, that limit their use. Therefore, significant efforts have been directed at providing lightweight components for electrochemical cells and electrochemical cell systems, such as fuel cell systems. Accordingly, there is a need for a lightweight generator of hydrogen gas for fueling fuel cells. It would be desirable to provide a hydrogen generator that is lightweight and portable, and adaptable for a variety of uses, including but not limited to PEM fuel cells. It would be further desirable to provide a hydrogen generator and related method that efficiently produces high quality hydrogen gas. It would be further desirable to have a hydrogen generator that can be accurately and easily controlled.